While C was adapted to the PDP-11, this was adding byte-level memory access. Otherwise I do no think there is anything in C specific to the PDP-11, or what would this be?
What makes C low-level is that it can work directly with the representation of objects in memory. This has nothing to do with CPU features, but with direct interoperability with other components of a system. And this is what C can do better than any other language: solve problems by being a part of a more complex system.
The post-increment and post-decrement operators mapped directly onto PDP-11 CPU addressing modes.
The integral promotion rules come directly from the PDP-11 CPU instruction set.
If I recall correctly so does the float->double promotions.
CPUs started adapting to C semantics around the mid-80's. CPU designers would profile C generated code and change to be able to more efficiently run it.
Thanks. I guess the integral promotion is related to byte-addressing. If you have bytes but can not directly do arithmetic on them, promoting them to word size seems natural.